Electrical measuring probes that measure the electrical conductivity of liquids are used, for example, to check the concentration of cleaning and disinfecting solutions in pressurized pipelines. In cases such as these, the electrical resistance of the product to be measured, i.e., the liquid, is generally measured. Where the liquid to be measured is of a chemically aggressive nature, those parts of the measuring probe which come into contact with it have to be sufficiently chemically resistant. The measuring probe also has to be designed in such a way that its accuracy of measurement does not suffer under the effect of variations in temperatures, for example, between 10.degree. and 95.degree. C.
The measuring probes are mostly made of machined, semi-finished thermoplastics, the electrical connection with the liquid to be measured being established by means of shaped elements of electrocarbon, fine steel, or noble metal. The seals between the electrically conductive shaped elements and the plastic parts are established, for example, by means of elastic sealing elements. In one such construction known from U.S. Pat. No. 4,227,151, several circular electrodes arranged concentrically are insulated from one another by intermediate layers of a thermoplastic.
Operational difficulties are encountered with measuring probes or measuring cells of the above kind due to the different coefficients of thermal expansion of the conductive metal and carbon elements on the one hand and the insulating plastic parts on the other hand. This results in cracking of the insulating plastic parts, separation at the interfaces, and deterioration in the bias of elastic sealing elements. The resulting leak at the interface between conductive and insulating parts leads both to insulating faults and hence to measurement errors and also to corrosion and destruction of the individual components, particularly the electrical power lead.
Accordingly, German published patent application (DE-OS) No. 30 06 877 proposes a measuring probe particularly designed to supply analog data, in which those parts of the electrical leads which have to be screened off from the liquid to be measured form a mechanically stabilizing insertion frame of an electrically insulating elastomeric sleeve which enters into a so-called rubber-to-metal bond with the constituent material of the leads and which determines the shape of the probe. Although this measuring probe represents an advance over the existing state of the art, it cannot be used in chemically aggressive or corrosive liquids such as nitric acid or caustic soda because known elastomers are not permanently stable under such working conditions. Another problem of the known probe lies in the considerable thermal stresses which still occur in the event of wide variations in temperature.